Is it permissible for a woman to perform 'umrah on behalf of her deceased father using the 'umrah of her non-pubescent children, or to perform 'umrah for herself and then perform 'umrah for her father from Makkah on the same trip, and does the reward for 'umrah on behalf of the deceased reach them if it takes place in Ramadan?

Scholars have differed regarding the permissibility of a minor performing Hajj or Umrah on behalf of another. The Hanafis permit it for both obligatory and supererogatory acts if the child is capable of worship. The Hanbalis forbid it absolutely because the child has not yet performed Hajj for themselves. The Shafi'is permit it for supererogatory acts but not for obligatory ones. As for the Malikis, they permit it on the condition that the person on whose behalf Hajj is performed has given a will. Your going to Tan'im after completing your Umrah to perform Umrah on behalf of your father is permissible and there is no harm in it, and the reward of the Umrah in Ramadan will reach him.